The most popular gas discharge tubes in use for displays are the types which use neon gas or a combination of argon and mercury gases. The neon gas when excited glows at a characteristic red color. The combination of argon and mercury gases when excited typically glow in a pale blue color. All other colors used in display signs are typically phosphor-coated tubes in which argon and mercury gases are placed. The argon-mercury vapors are excited which in turn cause the phosphors to glow. The phosphors then glow at the selected color.
Excitation power supplies for gas discharge tubes and in particular for neon or argon-mercury discharge tubes, have been known for many years. The most common form of a discharge supply is a neon light transformer having a 60 Hz, 120 volt AC primary with 60 Hz approximately 10 KVAC secondary which is directly connected to the electrodes attached to either end of the gas discharge tube. A transformer of this size tends to weigh 10-20 pounds due to the massive core, the number of primary and secondary windings and the potting of the transformer in a tar-like material to prevent arcing. This results in a very larger bulky and unsightly excitation supply.
More recently, light-weight switching power supplies have been used to step up the 60 Hz, 120 VAC voltage to a higher frequency for conversion to a higher voltage for exciting gas discharge tubes. In general, the higher switching frequency allows the use of smaller, more light-weight transformers. The switching frequency may be fixed or may be variable as described in U.S. Pat. No. 4,916,362 issued Apr. 10, 1990 entitled Excitation Supply for Gas Discharge Tubes to Edward D. Orenstein which is assigned to the same assignee of the present invention, and which is hereby incorporated by reference.
It is known in the art to create a "crawling effect" gas discharge tubes as described in U.S. Pat. No. 4,742,278, issued May 3, 1988 entitled Single Connection Gas Discharge Display and Driver to Robert E. Iannini. This crawling effect causes the excitation of the gas in the gas discharge tube to start at one end of the tube and grow to the other end. In U.S. Pat. No. 4,742,278, this effect is accomplished in gas discharge tubes having only a single connection. The growth of the excitation along the length of the tube is accomplished by ramping the voltage on the input of the single connection to the tube. In order to accomplish this single ended connection on the tube, the power source must be connected using a polarized plug to ensure that one side of the power supply is reference to earth ground. Floating supplies are specifically prohibited from this technique.
Also described in U.S. Pat. No. 4,742,278, is that means for effecting the crawling effect is by varying the voltage of a fixed frequency switching power supply. As described in U.S. Pat. No. 4,916,362, fixed frequency power supplies are limited in the types of gas discharge tubes that can be driven due to the "bubble effect". The bubble effect is caused by alternating light and dark sections a gas discharge tube due to fixed frequency harmonics for impedance mismatches between the driving power supply and the gas discharge tube. Thus a wide variety of gas discharge tubes cannot be used with a single fixed frequency supply due to the occasion of the bubble effect.
Therefore, there is a need in the prior art for a high frequency switching gas discharge tube supply which provides a "crawling effect" in gas discharge tubes containing neon, argon, mercury gases or other gases for a two electrode tube and which operates independent of the polarity of the line supply mains. There is also a need in the prior art for a swept frequency switching gas discharge tube supply which provides a "crawling effect" in gas discharge tubes by sweeping the excitation frequency of the drive signal.